1. Field of the Invention
The present invention relates to an optical apparatus having a focus detecting apparatus of a photographing camera, video camera, or the like and, more particularly, the invention is suitable for use in a single-lens reflex camera having a focus detecting apparatus in which a pupil of an objective lens is divided into a plurality of, for instance, two areas. Light intensity distributions regarding two object images are formed by using light fluxes passing through the two areas, a relative positional relation between the two light intensity distributions is obtained, and an in-focus state of the objective lens is thereby detected.
2. Related Background Art
Hitherto, what is called an image deviation system has been known as a focus detection system of the photo sensitive type using light fluxes which passed through an objective lens.
For instance, as already proposed by many Japanese Patent Applications such as JP-A-59-107311, JP-A-59-107313, etc., the image deviation system is constructed in a manner such that a field lens is arranged near a prospective focusing plane on which an object image is formed by an objective lens, a secondary optical system having a porous mask and one or two pairs of secondary lenses is arranged after the field lens, and photo sensitive means having a plurality of photosensitive element arrays is further arranged after the secondary optical system.
Light intensity distributions regarding two object images are respectively formed on the photo sensitive element array surfaces by using the light fluxes which had passed through two different areas of the pupils of the objective lens and came from the secondary optical system. The relative positional relation of the two light intensity distributions which are formed on the photo sensitive element array surfaces at (that time that is, the deviation amounts of the light intensity distributions) differ in dependence on the in-focus state of the objective lens. For instance, such deviation amounts appear as a lateral deviation amount of a secondary object image according to an out-of-focus amount of the object image from the prospective focusing plane of the objective lens in the arranging direction of the elements on the photo sensitive element array.
The out-of-focus state of the objective lens, that is, the focus deviation amount, is obtained by detecting the relative positional relation of the two light intensity distributions at that time, namely, the amount of lateral deviation of the light intensity distributions on the photo sensitive means.
Generally, there is a predetermined functional relation between the relative lateral deviation amount of the two light intensity distributions on the photo sensitive element array and the out-of-focus amount of the objective lens.
In most of the single-lens reflex cameras having the focus detecting apparatus of the image deviation system, a focus detecting apparatus is arranged in the bottom portion of the camera or on the roof type penta prism side. The light fluxes which passed through the objective lens are transmitted through or reflected by a half transparent mirror surface provided in a part of a quick return mirror and, thereafter, they are led to the focus detecting apparatus.
In this case, if the surface of the half mirror is slanted to the dividing direction of the pupils of the objective lens, the focus detecting light fluxes which are led to the focus detecting apparatus asymmetrically enter the half mirror surface, so that the incident angle of the light flux from each pupil of the objective lens to the half mirror surface largely differs depending on the pupil positions.
One of the problems due to such an optical arrangement has already been pointed out in U.S. patent application Ser. No. 115,905 and solved. On the other hand, if a metal mirror is used as a half mirror, an amount of light which is absorbed is large. Therefore, in many cases, a practical dielectric multilayer film is formed on a glass flat plate.
However, when the dielectric multilayer film is used as a half mirror surface, the spectral characteristics such as reflectivity, transmittance, and the like largely differ depending on the difference of incident angles.
Such a phenomenon has been known in, for instance, JP-A-57-179807 with respect to a dichroic mirror of a color television camera.
As a result of the positional differences of the spectral characteristics, an error occurs between a pair of light intensity distributions with respect to objects which are formed on two photo sensitive element array surfaces, causing the focus detecting accuracy to deteriorate.